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Description/Abstract

Extruded polyethylene (PE) is used as the bulk insulation for ac high voltage cables; because of its high electric resistivity and breakdown strength. Although the material at present has limited use in dc power cables, it is used extensively in submarine optical communication cable systems. This paper reports on the study of the short term characteristics and long term performance of LDPE insulation under dc electric stress. The results of an investigation is presented in which controlled defects as found in practical systems (voids, metallic and non-metallic particles) were introduced into well characterised polymer material so that their influence on electrical strength and breakdown mechanisms could be determined. Samples were compression-moulded under laboratory conditions and subjected to ramp to failure and electrical ageing tests at various stresses. Weibull statistics are used to analyse the results. Examination of electrically aged samples indicates an oxidation degradation around the defects due to stress enhancement. From the failed samples a value of n in the inverse power model was estimated. The residual life of aged samples was also examined and the results presented.